1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium. More specifically, the present invention relates to a perpendicular magnetic recording medium for use in energy-assisted magnetic recording systems.
2. Description of the Prior Art
A perpendicular magnetic recording system is employed as the technology for increasing the magnetic recording density. A perpendicular magnetic recording medium includes at least a non-magnetic substrate and a magnetic recording layer formed of a hard magnetic material. The perpendicular magnetic recording medium may further optionally include a soft magnetic underlayer formed of a soft magnetic material and playing a role in concentrating the magnetic flux of a magnetic head onto the magnetic recording layer, an underlayer for orienting the hard magnetic material of the magnetic recording layer in a target direction, a protective film for protecting the surface of the magnetic recording layer, and the like.
Japanese Patent Application Publication No. 2001-291230 (Patent Document 1), Japanese Patent Application Publication H8-083418 (Patent Document 2, and WO 2002/039433 (Patent Document 3) each describe a granular magnetic material as the material for forming a magnetic recording layer of a perpendicular magnetic recording medium. The granular magnetic material contains magnetic crystal grains and a non-magnetic substance segregated to surround the magnetic crystal grains. Individual magnetic crystal grains within the granular magnetic material are magnetically separated by the non-magnetic substance.
For the purpose of further improving the recording densities of perpendicular magnetic recording media, in recent years there is an urgent need for reduction of the grain diameters of the magnetic crystal grains contained in the granular magnetic materials. Reducing the grain diameters of the magnetic crystal grains, however, leads to a decrease in thermal stability of the recorded magnetization (signals). In order to compensate for the decline in thermal stability resulting from the reduction of the grain diameters of the magnetic crystal grains, the magnetic crystal grains in the granular magnetic materials need to be formed using materials with high magnetocrystalline anisotropies.
One of the materials proposed as a material with high magnetocrystalline anisotropy is an L10 ordered alloy. Japanese Patent No. 3318204 (Patent Document 4), Japanese Patent No. 3010156 (Patent Document 5), Japanese Patent Application Publication No. 2001-101645 (Patent Document 6), Japanese Patent Application Publication No. 2004-178753 (Patent Document 7), and Japanese Translation of PCT Application No. 2010-503139 (Patent Document 8) each suggest, as the L10 ordered alloy, FePt, CoPt, FePd, CoPd, or the like, an alloy that contains at least one type of element selected from the group consisting of Fe, Co, and Ni, and at least one type of element selected from the group consisting of Pt, Pd, Au, and Ir. Patent Documents 4-8 documents also describe various methods for producing L10 ordered alloy thin films.
On the other hand, because the film thickness of a magnetic recording layer is basically uniform in an in-plane direction of the medium, reducing the grain diameter of the magnetic crystal grains thereof means reducing the cross-sectional area of the magnetic crystal grains of a certain height. Therefore, a diamagnetic field acting on the magnetic crystal grains themselves becomes small, whereas a magnetic field required to reverse the magnetization of the magnetic crystal grains (reversal magnetic field) becomes large. In view of the shape of the magnetic crystal grains, the improvement of the recording density implies that a larger magnetic field is required for recording signals.
Energy-assisted magnetic recording systems such as a heat-assisted recording system and a microwave-assisted recording system have been proposed by Inaba et al., “New High-density Recording Technology—Energy-assisted Magnetic Recording Media”, Fuji Electric Journal, Fuji Electric Co., Ltd., Research (Non-patent Document 1) to improve the magnetic field strength required for signal recording. The heat-assisted recording system utilizes the temperature dependence of the magnetic anisotropy constant (Ku) of a magnetic material, which is a characteristic where the higher the temperature, the lower the Ku. This system uses a head that functions to heat a magnetic recording layer. In other words, this system executes writing while reducing a reversal magnetic field by increasing the temperature of the magnetic recording layer and temporarily reducing the Ku. The Ku returns to its original high value after the temperature of the magnetic recording layer drops, keeping the recorded signals (magnetization) stable. In the application of the heat-assisted recording system, a magnetic recording layer needs to be designed based on its temperature characteristics in addition to the conventional design guidelines.
According to a study by Igarashi et al., “Computer Simulation for Thermal Assist Recording—The Format of Technical Report”, Technical Report of IEICE, MR2004-39, 2004 (Non-patent Document 2), the transition width between recording bits in the heat-assisted recording system is determined based on a head magnetic field gradient and a temperature gradient.
In addition, Japanese Patent Application Publication No. 2012-48784 (Patent Document 9) discloses a method for forming a magnetic recording layer of a granular magnetic material such as L10 ordered alloy by repeatedly executing the cycle of heating and depositing a substrate, the granular magnetic material containing magnetic crystal grains and a non-magnetic substance. Repeating the cycle of heating and depositing is said to not only prevent a decline in temperature of each substrate while the granular magnetic material is being deposited, but also to reduce the average grain diameter of the magnetic crystal grains and improve the perpendicular orientation. Patent Document 9 also discloses a magnetic recording layer in which two types of layers with different non-magnetic substances (a FePtAg-C layer and a FePtAg-SiO2 layer) are stacked.
The inventor has discovered the need to increase the film thicknesses of magnetic recording layers in order to employ the energy-assisted magnetic recording systems. However, it was found out that when forming a magnetic recording layer using an ordered alloy granular magnetic material containing magnetic crystal grains and a non-magnetic substance, simply increasing the film thickness of the magnetic recording layer leads to degradation of the magnetic characteristics of the magnetic recording layer, such as the magnetic anisotropy constant (Ku) and the squareness ratio. For instance, in case of an ordered alloy granular magnetic material in which carbon (C) is used as a non-magnetic substance, the carbon, which is supposed to be present at grain boundaries of the magnetic crystal grains, covers the upper surface of the magnetic crystal grains to inhibit columnar growth of the magnetic crystal grains, which causes secondary growth of the magnetic crystal grains on the carbon in the upper surface, deteriorating the magnetic characteristics of the magnetic recording layer. When an oxide material is used as the non-magnetic substance, the magnetic characteristics decline as a result of increasing the film thickness of the magnetic recording layer.
The present invention was contrived in view of the foregoing problems, and an object thereof is to provide a magnetic recording medium that has a thick magnetic recording layer with excellent magnetic characteristics and a granular structure.